Display only sub-pages of ID on the ACF

custom_query_relationship_ACF.php

<?php 
// display only sub-pages of a TARGET_PAGE on ACF_NAME custom field.
// https://www.advancedcustomfields.com/resources/acf-fields-relationship-query/
// in functions.php

function childpages_query( $args, $field, $post_ID ) {      
  $args = array(
    'post_type'      => 'page',
    'posts_per_page' => -1,
    'post_parent'    => TARGET_PAGE_ID    
  );      

	return $args; 
} 

add_filter('acf/fields/relationship/query/name=ACF_NAME', 'childpages_query', 10, 3);
?>